A longitudinal study (3days through 8 week) of the histopathology of the middle ear mucosa and the cytology of the middle ear fluids were done in an experimental animal model (20 cats) of eustachian tubal occlusion and the following results were obtained. 1) Histopathological changes were noticed in all 38 ears and middle ear fluids were developed in 32 ears. 2) Histopathological findings inclued epithelial hyperplasia, metaplasia, subepithelial space widening, fibroblast proliferation and inflammatory cell infiltration. Epithelial changes were occurred after the second week and increased till the eighth week. Thickening or edema of the subepithelial space were begun at the third day and gradually increased thereafter. Fibroblast proliferation was appeared at the first week and increased thereafter. Inflammatory cells of the subepithelial space showed that polymorphonuclear leukocytes were appeared in the early periods and macrophages and lymphocytes in later periods. 3) New bone formation was not prominent and cement lines were noticed at some areas of bony promontory and anterior bulla wall after the fourth week. 4) As the cytological findings of middle ear effusion, the initial appearance of a higher percentages of polymorphonuclear leukocytes decreased from the first week after the obstruction and slight increase in macrophages was observed thereafter. The number of the appeared lymphocytes in the middle ear fluids was not changed during the whole period. In conclusion, this experimental model of otitis media with effusion was very useful for histopathological and cytological study of otitis media and with this, the pathogenesis of the disease will be elucidated and proper treatment and prevention could be followed.